As the display mode of liquid crystal display devices (LCD), twist nematic (TN) mode, super twist nematic (STN) mode, and active matrix (AM-LCD) mode have been proposed and used one after another in practice. Among these, the super twisted nematic (STN) mode which was proposed by T. J. Scheffer et al. (Appl. Phy. Lett., 45 (10), 1021 (1984)) and in which the orientation of liquid crystal molecules arranged between an upper and a lower substrates is twisted by 180.degree. to 270.degree. was adopted as LCD of personal computers and word processors, and the improvement in various characteristics of liquid crystal compositions have come to be required.
Following characteristics are required of liquid crystal compositions used for liquid crystal display devices of STN mode:
(1) Liquid crystal compositions exhibit a nematic liquid crystal phase at a wide temperature range including room temperature. Especially, liquid crystal compositions have a high nematic-isotropic phase transition temperature (clearing point).
(2) Threshold voltage of liquid crystal compositions is low to reduce the consumption of electric power.
(3) Viscosity (.eta.) of liquid crystal compositions is low to shorten response time (.tau.) as much as possible.
(4) Liquid crystal compositions have an appropriate optical anisotropy (.DELTA.n) depending on liquid crystal display devices
(5) Liquid crystal compositions are chemically stable.
Demand particularly for LCD which is characterized by having a small-size and light weight, and being portable (for example, notebook type personal computers) has recently increased. In order to meet with the increased demand, it is necessary to further expand the temperature range at which liquid crystal compositions exhibit a nematic liquid crystal phase, particularly to raise clearing point, to cope with the change in outside environmental temperature.
Further, it has strongly been required to cope with a display of dynamic pictures by means of LCD. In order to cope with the display of dynamic pictures, the response time of liquid crystal compositions at the time when liquid crystal devices are driven must be short. Since response time is proportional to the viscosity of liquid crystal compositions, the development of liquid crystal compositions having a low viscosity have been pursued.
It is an already well known means to repress the reverse twist of liquid crystal molecules and impart a constant helical structure of a righthanded rotation or lefthanded rotation to the molecules by adding a minor amount of an optically active substance to liquid crystal compositions thereby to maintain the quality of display. However, when the ability of the optically active substance added as chiral component in twisting the orientation of liquid crystal molecules is low, it is evident that a necessity of adding the chiral component in a comparatively high concentration arises to obtain a required helical pitch length, and other parameters of liquid crystal compositions are adversely affected. Further, generally known optically active substances are not preferable since the pitch length tend to become larger as temperature raises.
For instance, in a SBE (super-twisted birefringence) mode, the length of the pitch (P) of the orientation of liquid crystal molecules is varied with the change in temperature, and thus, the value of the ratio (P/d) of the pitch length (P) to the cell thickness (d) of a display device is varied. Whereas the P/d is usually adjusted to lower than 2, when the ratio becomes 2 or more with the change in temperature, the twist of 270.degree. of the orientation of liquid crystal molecules is changed to 90.degree..
Further, from the viewpoint of an improvement for increasing display capacity, a steepness is necessary in the change in transmittance of light when a voltage was increasingly applied to display devices. While G. Baur and W. Fehrenbach have reported at the 15th conference on liquid crystals held at Freiburg in 1985 the result of calculation that the steepness is considerably improved when the twist: was made to 270.degree., it is considered to be necessary even in the report to lower the dependency of the pitch length on temperature.
In Laid-open Japanese Patent Publication No. Sho 63-22893, optically active compounds having a negative temperature dependence with their pitch length, that is, the substances pitch length of which decrease as temperature raises are disclosed, and a method for obtaining liquid crystal compositions pitch length of which is not varied with the change in temperature by mixing an appropriate amount of the optically active compound with conventional optically active compounds having a positive temperature dependence is proposed as a means to solve the problems mentioned above. Further, in Laid-open Japanese Patent Publication Nos. Hei 1-96155 and Hei 2-48555, optically active compounds having a negative or an extremely small positive temperature dependency of the pitch length and having a high twisting ability are disclosed, and a general formula by which those optically active compounds are expressed include the chiral component A used in the present invention.
However, the optically active compounds disclosed in Laid-open Japanese Patent Publication No. Sho 63-22893 are low in their twisting ability and thus, they must be added in a high concentration to obtain a required pitch length. In Laid-open Japanese Patent Publication Nos. Hei 1-96155 and Hei 2-48555, the clearing point and viscosity of Liquid crystal compositions are not described at all, but only examples of mixtures with cyano type liquid crystalline compounds and temperature characteristics of the pitch length are described.
As discussed above, whereas diligent studies are being carried out on liquid crystal compositions for various purposes, it is a current state that new improvements are required all the time.